Recently, in the field of optical communications, researches have been intensively made on optical frequency devision multiplexing communication techniques for transmitting through a single optical fiber a plurality of sets of information at different wavelengths of light, in order to greatly increase the information transmission capacity. To attain such multiplexing communications, an optical multiplexer/demultiplexer is needed which multiplexes and demultiplexes a large number of light waves used.
The optical multiplexer/demultiplexer used for such applications is required to have the performance stated below.
First, using a large number of light waves with the narrowest possible wavelength spacing is effective in increasing the information transmission capacity, and therefore, the optical multiplexer/demultiplexer should be able to multiplex and demultiplex such a large number of light waves. For example, the multiplexer/demultiplexer is required to multiplex and demultiplex light waves with 100 GHz frequency spacing, which is equivalent approximately to 0.8 nm wavelength spacing in 1.55 .mu.m band.
The optical multiplexer/demultiplexer is also required to have satisfactory passband flatness in the vicinity of passing wavelength.
For example, where an inexpensive LD is used as a light source in constructing an optical frequency devision multiplexing communication system with a view to reducing costs, the oscillation wavelength of the light source is liable to change with time or due to variations in temperature or humidity of the environment in which the light source is used. If the oscillation wavelength of the light source undergoes such a change, a loss variation occurs when light is propagated through the optical multiplexer/demultiplexer in the system, by an amount corresponding to the wavelength change depending on the spectrum response of the multiplexer/demultiplexer. The loss variation not only deteriorates the loss uniformity among wavelengths to be multiplexed/demultiplexed but also the S/N ratio, and eventually increases the cost of constructing the system.
In view of the foregoing, the loss variation of the optical multiplexer/demultiplexer should preferably be as small as possible. The optical multiplexer/demultiplexer is therefore required to have a characteristic such that the loss variation is, for example, 1 dB or less, that is, 1 dB bandwidth is large.
As such optical multiplexer/demultiplexer, an arrayed waveguide grating type is disclosed in Unexamined Japanese Patent Publication (KOKAI) No. 8-122557.
FIG. 8 is a plan view schematically showing the optical multiplexer/demultiplexer. This multiplexer/demultiplexer has a substrate 1 on which are arranged one or a plurality of input waveguides 2, an input-side slab waveguide 3 connected to the input waveguide(s) 2, a diffraction grating 4 connected to the input-side slab waveguide 3 and comprising a plurality of arrayed channel waveguides 4a, an output-side slab waveguide 5 connected to the arrayed waveguide grating 4, and a plurality of output waveguides 6 connected to the output-side slab waveguide 5.
In this optical multiplexer/demultiplexer, the junction between the input waveguide 2 and the input-side slab waveguide 3 is formed as shown in FIG. 9.
Specifically, the input waveguide 2, which is surrounded by a cladding material 10 and has a path width W1, has a tapered end portion expanded in the width direction of the path, and a slit 7 is formed in the center of the tapered portion, thus defining two waveguide portions 2a and 2b of equal width. The input waveguide 2 is connected to the input-side slab waveguide 3 at the tapered portion, or the two waveguide portions 2a and 2b.
In the input waveguide 2 constructed in this manner, light propagated through the input waveguide 2 enters the input-side slab waveguide via the tapered portion. At this time, the two waveguide portions 2a and 2b of the tapered portion equivalently function as a core. Consequently, at a location just in front of the input-side slab waveguide 3, the electric field distribution of light is spread as a whole in the width direction and has a bimodal shape with two maximal values.
This optical multiplexer/demultiplexer is allegedly capable of attaining 3 dB bandwidth of about 0.8 nm with respect to about 1 nm wavelength spacing.
In the prior art device, however, almost no consideration is given to the passband flatness of light output from the output waveguides 6, or more specifically, to 1 dB bandwidth which is an important characteristic when the optical multiplexer/demultiplexer is applied to an actual optical frequency devision multiplexing communication system.
The inventors hereof therefore actually fabricated an optical multiplexer/demultiplexer as shown in FIGS. 8 and 9 and examined its spectrum response.
Specifically, an optical multiplexer/demultiplexer with silica-based waveguides was produced, wherein the input waveguide 2 had a path width W1 of 6.5 .mu.m, the connecting portion of the input-side slab waveguide 3 had a width W2 of 15.0 .mu.m, the trapezoidal slit 7 had a connection width CW of 1.0 .mu.m on the input waveguide 2 side and a connection width SW of 2.0 .mu.m on the slab waveguide 3 side, the tapered portion was tapered at an angle .theta. of 0.4.degree., and the waveguides had a relative index difference of 0.8% and a path height of 6.5 .mu.m, to derive light with 100 GHz wavelength spacing, that is, about 0.8 nm wavelength spacing in 1.55 .mu.m band. With light of 1.55 .mu.m band input to the input waveguide 2, the spectrum response was examined.
FIG. 10 shows the electric field distribution of light observed at a location just in front of the input-side slab waveguide 3, and FIG. 11 shows the spectrum response at the output waveguide 6.
In FIG. 10, the horizontal axis represents the width direction of the path at a location immediately in front of the input-side slab waveguide 3, and the position "0" indicates the center along the width direction, that is, the center point of the width W2 shown in FIG. 9. In FIG. 11, the horizontal axis represents wavelength of light propagated through the output waveguide 6, and the position "0" indicates the center wavelength of the propagated light.
To actually measure the electric field distribution, the fabricated optical multiplexer/demultiplexer must be destroyed, but in the experimentation, the electric field distribution was estimated/calculated by means of simulation according to beam propagation method (BPM), instead of destroying the device.
As is clear from FIG. 10, the electric field distribution showed a bimodal shape having maximal values a and b and a minimal value c therebetween. The spacing between the two maximal values a and b was 7.0 .mu.m and the ratio c/a was 0.59.
With regard to the spectrum response, 1 dB bandwidth, which is a wavelength range 1 dB higher than a minimum insertion loss, was found to be 0.37 nm, and 3 dB bandwidth was 0.50 nm.
In the aforementioned Unexamined Japanese Patent Publication No. 8-122557, it is stated that 3 dB bandwidth can be further increased by setting the ratio SW/W2 of the junction between the input waveguide and the input-side slab waveguide shown in FIG. 9 to 0.2 to 0.6.
The inventors therefore fabricated an optical multiplexer/demultiplexer with a junction having the same parameters as the aforesaid ones, except that the connection width SW of the junction shown in FIG. 9 was set to 3.0 .mu.m, and measured the electric field distribution and the spectrum response under the same conditions. The junction of the fabricated multiplexer/demultiplexer had an SW/W2 ratio of 0.2.
FIG. 12 shows the electric field distribution of light observed at a location just in front of the input-side slab waveguide, and FIG. 13 shows the spectrum response of light from the output waveguide.
In this device, the spacing between the maximal values a and b shown in FIG. 12 was 10.3 .mu.m, which is greater than the spacing (7.0 .mu.m) observed in the case of FIG. 10. However, the ratio c/a was 0.27, showing an increased difference between the maximal and minimal values in the electric field distribution. Also, in the spectrum response shown in FIG. 13, 3 dB bandwidth was 0.63 nm, which is greater than the value (0.5 nm) observed in the case of FIG. 11. However, the flatness of the output light was so poor that the insertion loss at the center wavelength was higher than the minimum loss by more than 1 dB, dividing 1 dB bandwidth into two with the center wavelength missing. Namely, 1 dB bandwidth failed to be widened.
The foregoing reveals the following.
(1) Where the electric field distribution of light at a location immediately in front of the input-side slab waveguide is made to have a bimodal shape with increased spacing between the maximal values a and b, 3 dB bandwidth of the spectrum response increases.
(2) If, in such a bimodal electric field distribution of light at a location immediately in front of the input-side slab waveguide, the difference between the maximal and minimal values is large, the spectrum response of the output waveguide shows increased insertion loss at the center wavelength, with the result that 1 dB bandwidth divides into two with no center wavelength.
The fact (2) poses a serious problem when the optical multiplexer/demultiplexer is used in constructing an optical frequency devision multiplexing communication system, as mentioned above.
Thus, although the optical multiplexer/demultiplexer disclosed in Unexamined Japanese Patent Publication No. 8-122557 is effective in widening 3 dB passing bandwidth, it is still associated with a problem that 1 dB bandwidth fails to be widened satisfactorily.
Also, in this optical multiplexer/demultiplexer, the slit structure formed at the junction between the input waveguide and the input-side slab waveguide for transforming an electric field distribution of the input light into bimodal shape has a Y-branch configuration constructed in closed space. It is therefore difficult to form the slit structure with high precision, thus lowering the yield during manufacture.
An object of the present invention is to provide an arrayed waveguide grating type optical multiplexer/demultiplexer which solves the aforementioned problems with the optical multiplexer/demultiplexer disclosed in Unexamined Japanese Patent Publication No. 8-122557 and whose 3 dB and 1 dB bandwidths can both be widened, compared with the prior art optical multiplexer/demultiplexer. Another object of the present invention is to provide an arrayed waveguide grating type optical multiplexer/demultiplexer which ensures high yield during manufacture, compared with the prior art optical multiplexer/demultiplexer.